Polypropylene resin compositions are extensively used as mechanical parts, automotive parts, etc. because they are generally excellent in chemical resistance and mechanical properties. As a result of the recent trend toward size increase and wall thickness reduction in various products in the pursuit of higher functions and higher profitability, there is a desire for polypropylene resin composition excellent in impact resistance, brittle temperature, rigidity, heat distortion resistance, and tensile elongation at break. Tensile elongation at break is one of the properties highly required of polypropylene resin compositions for use as an automotive material from the standpoint of, for example, preventing the material from breaking upon impact to scatter fragments, or enabling the material to absorb an impact through deformation or not to break in creeping. Heat distortion resistance also is one of the highly required properties from the standpoint of providing molded materials which do not deform in a high-temperature atmosphere when exterior automotive materials are coated in a coating line. Brittle temperature, tensile elongation at break, and impact resistance are properties inconsistent with rigidity and heat distortion resistance; namely, an improvement in the former or latter properties results in deterioration in the other. An invention has hence been desired which attains an improved balance among all these parties.
Unexamined Published Japanese Patent Application No. 3-188114 discloses a block copolymer comprising a polymer block formed from a vinylaromatic compound and a hydrogenated isoprene-butadiene block and having a quantity of heat of crystal fusion of 8 cal/g or small. However, this invention requires that the copolymer has an 1,2-bond content of 35% or lower, which gives a composition impaired especially in tensile elongation at break. Furthermore, the relationship between the quantity of heat of crystal fusion and the brittle temperature of a composition of the copolymer is not described or suggested therein. There also is no description of suggestion concerning order-disorder transition temperature or the heat distortion resistance of the composition. Consequently, the objects of the present invention cannot be accomplished with the technique disclosed therein.
POLYMER, Vol.38, No.17 (1997) describes a hydrogenated block copolymer comprising polystyrene and hydrogenated polybutadiene. Described therein are an example having a 1,2-bond content of 50 mol %, a styrene content of 20 wt % and a quantity of heat of crystal fusion of 5.1 J/g and an example having a 1,2-bond content of 40 mol %, a styrene content of 20 wt % and a quantity of heat of crystal fusion of 12.3 J/g. In this article, there is no description at all concerning effects of use of these hydrogenated block copolymers in a composition although mechanical properties of the copolymers are described. Furthermore, those values of the quantity of heat of crystal fusion for the hydrogenated block copolymer are outside of the range according to the present invention because the copolymers are produced through polymerization at an elevated temperature without removing the heat of reaction by cooling. It is therefore apparent that the objects of the present invention cannot be accomplished with the hydrogenated block copolymer described therein.
Unexamined Published Japanese Patent Application No. 8-20684 discloses, as a resin composition excellent in rigidity, heat distortion resistance, impact resistance and moldability, a resin composition comprising crystalline propylene, two hydrogenated block copolymers (a styrene-ethylene/butylene-styrene copolymer and a styrene-ethylene/propylene copolymer), an ethylene-.alpha.-olefin copolymer rubber and talc. However, there is no description or suggestion therein concerning the relationship in the hydrogenated block copolymers between the quantity of heat of crystal fusion and brittle temperature or between order-disorder transition temperature and heat distortion resistance. These properties of the disclosed block copolymers are still unsatisfactory.
As described above, neither a hydrogenated block copolymer capable of imparting an excellent balance between such properties as impact resistance, brittle temperature and tensile elongation at break and such properties as heat distortion resistance and rigidity nor a polypropylene resin composition having an excellent balance among these properties has yet been obtained.
An object of the present invention is to provide a hydrogenated block copolymer which makes it possible to provide a polypropylene resin composition having an excellent balance among impact resistance, brittle temperature, tensile elongation at break, rigidity and heat distortion resistance and further having excellent profitability.
Another object of the present invention is to provide a polypropylene resin composition having an excellent balance among the properties.